add additional dates to release date script

[pdurbin]

See "NEW" below.

• Write the script in pure Python with no dependencies.
• Work backward from the provided Release Date, which we prefer to be on a Wednesday.
• Curation Team Review starts on the Monday prior. After the date, put "(allocate 5 full days)".
• Code Freeze starts on the Thursday prior.
• Core PR Last Call starts on the Thursday prior.
• Community PR Last Call starts on the Thursday prior.
• NEW: Review Priorities on the Thursday prior.
• Start starts three months before the Release Date, on a Thursday. It's ok to put "??" for the sprint number.
• NEW: Define Release (show plan for the release) two weeks before start.

Here's some example output:

• Start: Sprint ??, 2026-03-26
• Community PR Last Call: 2026-05-21
• Core PR Last Call: 2026-05-28
• Code Freeze: 2026-06-04
• Curation Team Review: 2026-06-08 (allocate 5 full days)
• Release Date: 2026-06-17